The present invention relates to improvements in hydrodynamic-mechanical drives for buses, fork lifts or other types of vehicles. More particularly, the invention relates to improvements in torque converters and other parts of hydrodynamic-mechanical drives for vehicles.
U.S. Pat. No. 3,202,018 to Hilpert discloses a power transmission which includes a so-called direct or triloc converter. The converter comprises a customary driving member (pump), a driven member (turbine) which is designed for substantially centripetal flow of fluid therethrough, and a stator (guide blade ring) which is installed in the radially inner region of the converter chamber and is designed for axial flow of fluid therethrough. During traction operation, the pump and turbine of the converter rotate in the same direction. If during travel of the vehicle, the reversing gearing of the transmission is changed over, the turbine rotates counter to its normal direction of rotation. Thereby and by causing the input clutch between the prime mover and the pump of the converter to operate with a certain amount of slip, the converter can perform a hydrodynamic braking operation. A drawback of the just described transmission is that, during hydrodynamic braking, friction between the primary and secondary portions of the input clutch is extremely high. This is due to the fact that, in the converter of Hilpert, the specific pump torque is extremely high in the so-called counter-braking range. Furthermore, the turbine torque is also excessively high during the major part of the counterbraking range so that the corresponding friction clutch of the reversing gearing must operate with a sustantial amount of slip during hydrodynamic braking, i.e., friction between the primary and second parts of the friction clutch is extremely high. Therefore, the reversing gearing of the patented transmission must employ a powerful friction clutch whose parts must be cooled by a complex, expensive and bulky cooling system. The situation is further aggravated due to the fact that, during certain stages of the counterbraking range (i.e., at certain values of the ratio n.sub.T /n.sub.P of turbine and pump speeds), the specific turbine torque fluctuates within a wide range or varies at a totally unpredictable rate.
U.S. Pat. No. 3,749,209 to Weinrich et al. discloses a modified drive which employs a counter-rotating torque converter. The latter is, like that of Hilpert, used selectively for traction or for hydrodynamic braking of the vehicle. It is further known to provide a vehicle drive with a counter-rotating torque converter and an input clutch which transmits torque from a prime mover (e.g., a variable-speed engine) to the pump of the converter. However, in each such conventional drive, hydrodynamic braking invariably takes place only when the converter pump is idle, namely when the pump is arrested by a so-called pump brake. Therefore, hydrodynamic braking torque which is absorbed by the turbine decreases very pronouncedly as the speed of the turbine decreases. Consequently, it is impossible to bring the vehicle at least close to a full stop by hydrodynamic braking alone. In this respect, the braking action of conventional drives resembles that of a flow brake. Also, a desired braking torque can be selected only by changing the extent to which the converter chamber is filled with fluid whenever the turbine of the converter rotates at a high speed.